This invention relates to an electrical brush and more particularly to a stranded brush having each strand coated with an insulating material.
Electrical brushes are utilized in electrical machinery to transfer current between moving portions of the machine and stationary portions thereof and are normally made of monolithic slabs of carbon or composites of carbon and high conductive metals. In the early stages of development of electrical machinery stranded wire was gathered together in bundles, which resembled a paint brush, and utilized to transfer current between the stationary and moving parts of the electrical machinery, hence, were given the name brushes, a name which continued to be utilized even though the brush changed from a stranded structure to a monolithic structure.
The efficiency of high-current low-voltage DC machinery depends to a large measure on the performance of the brush systems, which transfer current from the rotating to stationary portions of the machine. In order to reduce the resistance losses and improve the overall efficiency of these systems, sintered metallic graphite brushes containing 50 to 75% of silver or copper have replaced conventional carbon or electrographic brushes. These brushes have about one-tenth the resistance of the conventional carbon brush; however, the low resistance in conjunction with bar leakage inductance creates a switching problem at the trailing edge of the brush zone where rotor bars break contact. This problem is known as metal depletion, a condition which occurs due to a high temperature rise at the interface surface where the brush leaves the bar, the temperature rise being sufficient to melt metal from the metal graphite composite brush structure. Depletion occurs first at the trailing edge of the brush zone where the power density reaches a maximum and then moves from the trailing edge toward the undepleted region. Thus, in effect the electrical trailing edge of the brush moves away from the physical trailing edge into the brush face. This continues to occur until power dissipated within the high resistance depletion zone becomes an appreciable fraction of the total power dissipated during the switching interval. At this point the depletion zone stabilizes at a fixed distance from the trailing edge of the brush.